Either the front wheels or the rear wheels of a front wheel- and rear wheel-drive vehicle are driven by an engine and the other thereof are driven by an electric motor. A detecting device detects a coefficient of friction on a road surface during running of the vehicle. Control circuit controls to operate the electric motor when the vehicle is subsequently started in a condition in which the coefficient of friction on the road surface is equal to or smaller than a predetermined value. The detecting device for detecting the coefficient of friction on the road surface is made up of a section for detecting the driving force of wheels to be driven by the engine, a section for detecting a slip ratio of the wheels, and a section for obtaining the coefficient of friction on the road surface based on a correlation between the driving force and the slip ratio.